Field of the Disclosure
The present disclosure relates to ionizing radiation detection apparatuses of an electron tracking type.
Description of the Related Art
An advanced Compton method is known as a conventional method for detecting a γ-ray. In the stated method, the incident direction of an incident γ-ray is calculated with the use of the energy and the scattered direction vector of a scattered γ-ray produced through Compton scattering as well as the energy and the recoil direction vector of a recoil electron produced through the Compton scattering.
Nuclear Science Symposium Conference Record (NSS/MIC 2010) discloses a time projection chamber (TPC), which is a γ-ray detection apparatus that utilizes an advanced Compton method. The TPC is filled with a gas serving as a scatterer, and a planar electron collector (μ-PIC) that multiplies an ionization electron and detects multiplied ionization electrons is disposed inside the TPC. A recoil electron produced through Compton scattering travels while successively ionizing gas molecules and produces an electron cloud formed of a number of ionization electrons in its trajectory. This electron cloud is subjected to the force of an electric field in an electron drift region and drifts to the electron collector while retaining substantially the same shape as the trajectory of the recoil electron. The electron collector carries out gas electron multiplication through an electron avalanche effect and detects the projection position of the electron cloud (trajectory) on a two-dimensional plane.
Japanese Patent Laid-Open No. 2010-078319 discloses a radiation gas monitor that corrects a gain variation arising in part from a deterioration over time of a scintillator in a radiation detector.
A secondary electron ionized by a recoil electron is multiplied by a gas electron multiplier, but the gas electron multiplication factor varies as an outgassed substance from an inner surface or an internal structure of the gas chamber is mixed thereinto or as a quencher gas decomposes and deteriorates. Accordingly, the accuracy in determining the position (direction) of an incident γ-ray calculated on the basis of the detected energy of the recoil electron is reduced.
The energy of a calibration radiation source used in Japanese Patent Laid-Open No. 2010-078319 is higher than the energy of the source for measurement radiation. When the calibration radiation source emits a β-ray, a low-energy secondary electron can be mixed into a measurement energy region and detected as noise; and when the calibration radiation source emits a γ-ray, a low-energy scattered γ-ray produced through Compton scattering or a low-energy secondary electron can be mixed into the measurement energy region and detected as noise.